Intragranular strain field in columnar ice during elasto-viscoplatic transient creep regime, and relation with the local microstructure
Abstract
The viscoplastic behaviour of polycrystalline ice is strongly affected by the very strong anisotropy of ice crystals. Indeed, in the dislocations creep regime relevant e.g. for ice sheet flow, dislocation glide on the basal plane of ice single crystals leads to strain-rates ∼6 order of magnitude larger than strain-rates that might be obtain if only non-basal glide is activated. At the polycrystal scale, this behaviour is responsible for a strong mechanical interaction between grains in the secondary (stationary) creep regime, and strain-rate is essentially partitioned between soft grains well-oriented for basal glide and hard grains exhibiting an unfavourable orientation for basal slip. As a consequence, the macroscopic flow stress at the polycrystal scale essentially depends on the resistance of the hardest slip systems or on the associated accommodation processes such as climb of basal dislocation on non-basal planes. One therefore expects very strong strain localization in polycrystalline ice in this viscoplastic regime. On the other hand, during transient effects, elasticity comes in plays. But since elasticity of ice single crystal is almost isotropic, very different strain localizations are expected in purely elastic and purely viscoplastic deformation regimes. Consequently, strain-rate decreases by several orders of magnitude during the transient creep of polycrystalline ice. This effect is associated to stress redistribution between hard and soft grains, and is probably of great importance e.g. to understand transient regimes such as tide effects on ice shelves or on icy planets. It can be described by the coupling between elastic and viscoplastic responses, and the associated long-term memory effect. In view of a better understanding of such effects, and development of adapted micromechanical models, we are engaged in the measurement of intragranular strain field and field heterogeneities is columnar ices deformed under loading involving stress increments (recovery tests) in order to favour transient effects. For doing this, we make use a Digital Image Correlation technique with spatial resolution far smaller than the actual grain size. First results will be shown and interpreted with respect to the underlying microstructure (crystallographic orientation and spatial arrangement of grains).
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMMR33B1672G
- Keywords:
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- 0738 CRYOSPHERE / Ice;
- 3909 MINERAL PHYSICS / Elasticity and anelasticity;
- 5112 PHYSICAL PROPERTIES OF ROCKS / Microstructure;
- 5120 PHYSICAL PROPERTIES OF ROCKS / Plasticity;
- diffusion;
- and creep